Thickened aqueous bleach compositions are useful as bleaching solutions, disinfectants, hard surface cleaners and automatic dishwasher formulations. The problem with such bleach compositions, however, is that they suffer degradation and viscosity loss caused by chlorine at elevated temperatures.
Alkali soluble polyacrylate and polymethacrylate thickeners for aqueous solutions are well known. However, these have poor long term stability in alkaline, oxidative solutions such as cleaning formulations containing hypochlorite. EP 541203 describes a hypochlorite bleach containing automatic dishwashing gel thickened with CARBOPOL crosslinked polyacrylic acid. The function of the polyacrylate thickener in the gel is to expand and bind the water. U.S. Pat. No. 5,348,682 describes a CARBOPOL polyacrylic acid or ACRYSOL ICS-1 (polyEA/MAA) bleach thickening composition. EP 636689 describes a bleach composition containing a halogen or peroxy bleach material, surfactant, and a non-crosslinked polymer. The polymeric thickener is prepared from a charged hydrophilic monomer and an uncharged hydrophobic monomer. However, those skilled in the art will recognize that this formulation will not have long term stability as measured by the accelerated aging tests.
U.S. Pat. No. 5,169,552 and U.S. Pat. No. 5,384,061 describes the addition of benzoic acid or its derivatives as radical scavengers to prevent viscosity drop during accelerated aging by protecting CARBOPOL resins from oxidation by the hypochlorite. U.S. Pat. No. 4,867,896 describes CARBOPOL analogues that maintained the formulation viscosity solution by replacing the multifunctional sucrose allyl ether and pentaerythritol allyl ester crosslinkers with crosslinkers such as divinyl benzene or 1,2,4-trivinyl cyclohexane which are relatively inert to degradation by alkaline hypochlorite.
U.S. Pat. No. 4,839,077 describes the use of mixed surfactant systems to build viscosity of hypochlorite solutions wherein the addition of small amounts of ethylene/acrylic acid polymer causes a synergistic viscosity increase greater than can be obtained by surfactant thickening alone. The polymer is relatively low molecular weight, not cross-linked and has poor solubility requiring the presence of surfactants, particularly a nonionic such as amine oxide, to be soluble. These formulations have moderate heat aging stability as shown by a 50% loss in viscosity after 4 weeks at 100.degree. F.
EP 636691 describes the use of a non-crosslinked styrene-methacrylic acid polymer to thicken hypochlorite/surfactant solutions wherein cross-linking the polymer gave higher viscosity but decreased clarity of the solution. These thickened solutions lacked long term stability loosing 50% of their viscosity after 6 weeks at room temperature.
For these reasons, there continues to be a need for a controlled release bleach thickening composition which maintains a thickening effect even at temperatures approaching 120.degree. F. for three to four weeks.